Battery assembly with multi-function structural assembly

ABSTRACT

A battery assembly according to an exemplary aspect of the present disclosure includes, among other things, a first cell stack including a plurality of battery cells and a structural assembly including a first pocket sized and shaped to receive the first cell stack. The structural assembly is configured to assert a compressive load on the first cell stack and at least partially enclose the first cell stack.

TECHNICAL FIELD

This disclosure relates to a battery assembly for an electrifiedvehicle. The battery assembly includes a structural assembly configuredto retain, enclose and/or thermally manage a plurality of battery cells.

BACKGROUND

The need to reduce automotive fuel consumption and emissions is wellknown. Therefore, vehicles are being developed that either reduce orcompletely eliminate reliance on internal combustion engines.Electrified vehicles are one type of vehicle being developed for thispurpose. In general, electrified vehicles differ from conventional motorvehicles because they are selectively driven by one or more batterypowered electric machines. Conventional motor vehicles, by contrast,rely exclusively on the internal combustion engine to propel thevehicle.

High voltage battery assemblies are employed to power the electricmachines of electrified vehicles. The battery assemblies include cellstacks constructed of a plurality of battery cells. An array structurebinds the battery cells of each cell stack. A separate enclosureassembly houses and seals the battery cells from the exteriorenvironment. Yet another separate structure, typically configured as acold plate, is commonly positioned in contact with the battery cells tothermally manage the heat generated by the cells.

SUMMARY

A battery assembly according to an exemplary aspect of the presentdisclosure includes, among other things, a first cell stack including aplurality of battery cells and a structural assembly including a firstpocket sized and shaped to receive the first cell stack. The structuralassembly is configured to assert a compressive load on the first cellstack and at least partially enclose the first cell stack.

In a further non-limiting embodiment of the foregoing battery assembly,the plurality of battery cells are individual cells disposedside-by-side and unbound relative to one another.

In a further non-limiting embodiment of either of the foregoing batteryassemblies, each of the plurality of battery cells is contiguous with atleast one wall of the structural assembly.

In a further non-limiting embodiment of any of the foregoing batteryassemblies, a second cell stack is received within a second pocket ofthe structural assembly.

In a further non-limiting embodiment of any of the foregoing batteryassemblies, a wall of the structural assembly separates the first pocketfrom the second pocket.

In a further non-limiting embodiment of any of the foregoing batteryassemblies, the structural assembly includes a plurality of walls thatare joined together.

In a further non-limiting embodiment of any of the foregoing batteryassemblies, at least one of the plurality of walls includes a channelconfigured to communicate a fluid to thermally condition the pluralityof battery cells.

In a further non-limiting embodiment of any of the foregoing batteryassemblies, a bus bar module is positioned over top of the first cellstack.

In a further non-limiting embodiment of any of the foregoing batteryassemblies, a base is positioned at a bottom of the structural assemblyand a cover is positioned at a top of the structural assembly.

In a further non-limiting embodiment of any of the foregoing batteryassemblies, a resilient envelope is disposed around an entire perimeterof the structural assembly.

A battery assembly according to another exemplary aspect of the presentdisclosure includes, among other things, a cell stack and a structuralassembly at least partially surrounding the cell stack, the structuralassembly including a plurality of walls each including at least onechannel configured to communicate a fluid to thermally condition thecell stack.

In a further non-limiting embodiment of the foregoing battery assembly,the structural assembly includes a first wall having a first channel ofa first cross-sectional area and a second wall having a second channelof a second cross-sectional area greater than the first cross-sectionarea.

In a further non-limiting embodiment of either of the foregoing batteryassemblies, the cell stack includes a plurality of battery cells thatare unbound to one another prior to insertion into a pocket of thestructural assembly.

In a further non-limiting embodiment of any of the foregoing batteryassemblies, the structural assembly is configured to assert acompressive load on the cell stack after insertion of the cell stackinto the pocket.

In a further non-limiting embodiment of any of the foregoing batteryassemblies, the structural assembly is configured in a figure-eightshape.

A method according to another exemplary aspect of the present disclosureincludes, among other things, compressing a cell stack of a batteryassembly and inserting the cell stack into a pocket of a structuralassembly. The cell stack is unbound prior to insertion into the pocketand the structural assembly is configured to apply a compressive loadagainst the cell stack after insertion into the pocket.

In a further non-limiting embodiment of the foregoing method, thecompressing step includes disposing a plurality of battery cells of thecell stack between opposing end spacers and applying a force to the cellstack at the opposing end spacers.

In a further non-limiting embodiment of either of the foregoing methods,the structural assembly is configured to at least partially enclose thecell stack.

In a further non-limiting embodiment of any of the foregoing methods,the structural assembly is configured to thermally manage a plurality ofbattery cells of the cell stack.

In a further non-limiting embodiment of any of the foregoing methods,the method includes sealing the cell stack of the battery assemblyrelative to an exterior environment after the inserting step.

The embodiments, examples and alternatives of the preceding paragraphs,the claims, or the following description and drawings, including any oftheir various aspects or respective individual features, may be takenindependently or in any combination. Features described in connectionwith one embodiment are applicable to all embodiments, unless suchfeatures are incompatible.

The various features and advantages of this disclosure will becomeapparent to those skilled in the art from the following detaileddescription. The drawings that accompany the detailed description can bebriefly described as follows.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates a powertrain of an electrified vehicle.

FIG. 2 is a cross-sectional view of a battery assembly according to afirst embodiment of this disclosure.

FIG. 3 is an exploded view of a structural assembly of a batteryassembly.

FIG. 4 is an exploded view of selected portions of a battery assembly.

FIGS. 5A and 5B illustrate additional configurations of a structuralassembly of a battery assembly.

FIGS. 6A and 6B illustrate additional features of the battery assemblyof FIG. 4.

FIG. 7 illustrates a battery assembly according to another embodiment ofthis disclosure.

DETAILED DESCRIPTION

This disclosure details a battery assembly for an electrified vehicle.The battery assembly may include one or more cell stacks each having aplurality of individual battery cells positioned adjacent to oneanother. A structural assembly of the battery assembly includes pocketsthat are sized and shaped to receive the cell stacks. In someembodiments, the structural assembly is configured to assert acompressive load against each cell stack and at least partially enclosethe cell stacks. In other embodiments, the structure assembly isconfigured to thermally condition the battery cells of each cell stack.The multi-function structural assembly reduces the number and size ofthe components of the battery assembly, substantially eliminatesconventional array retention components, and substantially eliminatesthreaded fastener connections to render a near zero air volume batteryassembly. These and other features are discussed in greater detail inthe following paragraphs of this detailed description.

FIG. 1 schematically illustrates a powertrain 10 for an electrifiedvehicle 12. Although depicted as a hybrid electric vehicle (HEV), itshould be understood that the concepts described herein are not limitedto HEV's and could extend to other electrified vehicles, including, butnot limited to, plug-in hybrid electric vehicles (PHEV's), batteryelectric vehicles (BEV's) and fuel cell vehicles.

In one embodiment, the powertrain 10 is a power-split powertrain systemthat employs a first drive system and a second drive system. The firstdrive system includes a combination of an engine 14 and a generator 18(i.e., a first electric machine). The second drive system includes atleast a motor 22 (i.e., a second electric machine), the generator 18,and a battery assembly 24. In this example, the second drive system isconsidered an electric drive system of the powertrain 10. The first andsecond drive systems generate torque to drive one or more sets ofvehicle drive wheels 28 of the electrified vehicle 12. Although apower-split configuration is shown, this disclosure extends to anyhybrid or electric vehicle including full hybrids, parallel hybrids,series hybrids, mild hybrids or micro hybrids.

The engine 14, which in one embodiment is an internal combustion engine,and the generator 18 may be connected through a power transfer unit 30,such as a planetary gear set. Of course, other types of power transferunits, including other gear sets and transmissions, may be used toconnect the engine 14 to the generator 18. In one non-limitingembodiment, the power transfer unit 30 is a planetary gear set thatincludes a ring gear 32, a sun gear 34, and a carrier assembly 36.

The generator 18 can be driven by the engine 14 through the powertransfer unit 30 to convert kinetic energy to electrical energy. Thegenerator 18 can alternatively function as a motor to convert electricalenergy into kinetic energy, thereby outputting torque to a shaft 38connected to the power transfer unit 30. Because the generator 18 isoperatively connected to the engine 14, the speed of the engine 14 canbe controlled by the generator 18.

The ring gear 32 of the power transfer unit 30 may be connected to ashaft 40, which is connected to vehicle drive wheels 28 through a secondpower transfer unit 44. The second power transfer unit 44 may include agear set having a plurality of gears 46. Other power transfer units mayalso be suitable. The gears 46 transfer torque from the engine 14 to adifferential 48 to ultimately provide traction to the vehicle drivewheels 28. The differential 48 may include a plurality of gears thatenable the transfer of torque to the vehicle drive wheels 28. In oneembodiment, the second power transfer unit 44 is mechanically coupled toan axle 50 through the differential 48 to distribute torque to thevehicle drive wheels 28.

The motor 22 can also be employed to drive the vehicle drive wheels 28by outputting torque to a shaft 52 that is also connected to the secondpower transfer unit 44. In one embodiment, the motor 22 and thegenerator 18 cooperate as part of a regenerative braking system in whichboth the motor 22 and the generator 18 can be employed as motors tooutput torque. For example, the motor 22 and the generator 18 can eachoutput electrical power to the battery assembly 24.

The battery assembly 24 is an exemplary electrified vehicle battery. Thebattery assembly 24 may include a high voltage traction battery packthat includes a plurality of battery cells capable of outputtingelectrical power to operate the motor 22 and the generator 18, amongother components. Other types of energy storage devices and/or outputdevices can also be used to electrically power the electrified vehicle12.

In one non-limiting embodiment, the electrified vehicle 12 has two basicoperating modes. The electrified vehicle 12 may operate in an ElectricVehicle (EV) mode where the motor 22 is used (generally withoutassistance from the engine 14) for vehicle propulsion, thereby depletingthe battery assembly 24 state of charge up to its maximum allowabledischarging rate under certain driving patterns/cycles. The EV mode isan example of a charge depleting mode of operation for the electrifiedvehicle 12. During EV mode, the state of charge of the battery assembly24 may increase in some circumstances, for example due to a period ofregenerative braking. The engine 14 is generally OFF under a default EVmode but could be operated as necessary based on a vehicle system stateor as permitted by the operator.

The electrified vehicle 12 may additionally operate in a Hybrid (HEV)mode in which the engine 14 and the motor 22 are both used for vehiclepropulsion. The HEV mode is an example of a charge sustaining mode ofoperation for the electrified vehicle 12. During the HEV mode, theelectrified vehicle 12 may reduce the motor 22 propulsion usage in orderto maintain the state of charge of the battery assembly 24 at a constantor approximately constant level by increasing the engine 14 propulsion.The electrified vehicle 12 may be operated in other operating modes inaddition to the EV and HEV modes within the scope of this disclosure.

FIG. 2 illustrates a battery assembly 24 that could be employed withinan electrified vehicle. For example, the battery assembly 24 could beemployed within the electrified vehicle 12 of FIG. 1. The batteryassembly 24 includes a plurality of battery cells 58 for supplyingelectrical power to various components of the electrified vehicle 12.Although a specific number of battery cells 58 are illustrated in thevarious Figures of this disclosure, the battery assembly 24 couldinclude any amount of cells. In other words, this disclosure is notlimited to the specific configuration shown in FIG. 2.

The battery cells 58 may be stacked side-by-side relative to one another(into the page in the cross-sectional view of FIG. 2) to construct oneor more cell stacks 62 (i.e., groupings of battery cells). In oneembodiment, the battery assembly 24 includes two cell stacks 62.However, the battery assembly 24 could include a single cell stack 62 ormultiple cell stacks 62 within the scope of this disclosure.

In one embodiment, the battery cells 58 are prismatic, lithium-ioncells. However, battery cells having other geometries (cylindrical,pouch, etc.) and/or chemistries (nickel-metal hydride, lead-acid, etc.)could alternatively be utilized within the scope of this disclosure.

The battery assembly 24 may additionally include a multi-functionstructural assembly 64. For example, the structural assembly 64 may beconfigured to apply a compressive load against the cell stacks 62, atleast partially enclose and seal the cell stacks 62, support andseparate the cell stacks 62 from one another, and thermally manage thebattery cells 58 of each cell stack 62. These functions are discussed ingreater detail below.

A base 66 may be attached to a bottom of the structural assembly 64 anda cover 68 may be attached to the top of the structural assembly 64. Thestructural assembly 64, the base 66 and the cover 68 combine to enclosethe cell stacks 62 such that the cell stacks 62 are substantially sealedfrom an exterior environment EX.

The battery assembly 24 may additionally include one or more bus barmodules 70. The bus bar modules 70 may be located above each cell stack62 and are adapted to electrically connect the battery cells 58 of eachcell stack 62. In one embodiment, the bus bar modules 70 are disposedbetween the cell stacks 62 and the cover 68.

FIG. 3, with continued reference to FIG. 2, illustrates a non-limitingembodiment of the structural assembly 64 of the battery assembly 24. Thestructural assembly 64 includes a plurality of walls 72. The walls 72may be extruded, cast, molded or manufactured using some other knowntechnique. In one embodiment, the walls 72 are made of aluminum,although other materials are also contemplated within the scope of thisdisclosure. The plurality of walls 72 may be formed into a desired sizeand shape and joined together, such as by welding, to construct therigid structural assembly 64. In one non-limiting embodiment, the walls72 are joined together to establish a figure-eight shaped design capableof receiving two cell stacks 62 (see FIG. 4). However, other designs arealso contemplated, including but not limited to designs in which thestructural assembly 64 is configured to receive a single cell stack 62(see FIG. 5A) or multiple cell stacks 62 (see FIG. 5B).

Each of the plurality of walls 72 may optionally include one or morechannels 74 that extend inside the walls 72. In one embodiment, thechannels 74 are openings (which may be machined, cast, formed, punched,extruded, etc.) formed either partially or entirely through the walls72. Other manufacturing techniques could be utilized to form thechannels 74. A fluid F may be communicated inside the channels 74 tothermally condition the battery cells 58 of each cell stack 62. Thefluid F may be a liquid, such as refrigerant, water, or ethylene glycolmixture, or a gas, such as air.

The channels 74 can be configured in different sizes and shapes to helpmeter and balance the flow of the fluid F through the walls 72. The sizeand shape of each channel 74 and the total number of channels 74provided are not intended to limit this disclosure. Depending on thedesign of the structural assembly 64, the fluid F may flow linearlywithin the channels 74 of one wall 72 and may turn at a right angle toflow into other walls 72. In the illustrated embodiment, the channels74-C of the center wall 72-C include a greater cross-sectional area thanthe channels 74 of the other walls 72 because the center wall 72-C ispositioned between two cell stacks 62 and will therefore require morefluid to achieve similar heat flux capabilities between the adjacentcell stacks 62.

The structural assembly 64 may additionally include an inlet cap 76 andan outlet cap 78. The inlet cap 76 and the outlet cap 78 may beconnected to at least one of the walls 72 to provide an inlet and anoutlet for receiving and expelling the fluid F. The inlet cap 76 and theoutlet cap 78 may be sized to receive a sufficient amount of the fluid Fto feed the other walls 72 and expel the fluid F from the walls 72. Thewalls 72 that receive the inlet cap 76 and the outlet cap 78 may alsoinclude additional end caps 80 for closing-off the channels 74 so thefluid F can only exit the structural assembly 64 via the outlet cap 78.

FIGS. 4, 6A and 6B schematically illustrate assembly of the batteryassembly 24. Referring first to FIG. 3, the battery cells 58 of a firstcell stack 62A may optionally be disposed between end spacers 82.Although not shown, additional spacers may optionally be positionedbetween each battery cell 58 to provide electrical isolation between theadjacent battery cells 58 of the first cell stack 62A. At this stage ofthe assembly, the battery cells 58 are unbound relative to one anotherby mechanical fastening devices such as threaded fasteners, brackets,plates and/or straps.

The battery cells 58 of the first cell stack 62 may be compressed, suchas with tooling (not shown). In one embodiment, the battery cells 58 arecompressed enough to lift and manipulate the first cell stack 62Awithout the battery cells 58 dropping out by applying a force F at eachend spacer 82. The compressed first cell stack 62A is then inserted intoa first pocket 84A of the structural assembly 64. The first cell stack62A may be slightly over-compressed such that it fits into the firstpocket 84A. Once the first cell stack 62 is received within the firstpocket 84A, the walls 72 of the structural assembly 64 exert acompressive load on the first cell stack 62A and at least partiallyenclose the first cell stack 62A. The battery cells 58 are contiguouswith at least one of the walls 72 of the structural assembly 64 oncereceived within the first pocket 84A (see FIG. 6A). The end spacers 82may include slots 86 that can be engaged by tooling for lifting andmanipulating the first cell stack 62A. The slots 86 may be filled afterthe tooling has inserted the cell stack 62A and been removed in order tosupport the battery cells 58 in a more uniform manner and promote a moreuniform opportunity for heat transfer at all portions of the batterycells 58 and the walls 72. The end spacers 82 may be made of a materialhaving a relatively low co-efficient of friction that facilitatessliding against the walls 72 for simplifying insertion of the first cellstack 62A into the first pocket 84A of the structural assembly 64. Inone non-limiting embodiment, the end spacers 82 are made of ultrahighmolecular weight polypropylene (UHMWPP).

Alternatively, the slots 86 may be omitted, thus leaving end spacers 82as a more continuous sheet. The first cell stack 62A may be compressedto fit into the pocket 84A. An independent pusher block may be used toslide the first cell stack 62A into the pocket 84A.

If the structural assembly 64 includes additional pockets, additionalcell stacks 62 may be received therein. For example, as shown in FIGS.6A and 6B, a second cell stack 62B may be inserted into a second pocket84B of the structural assembly 64.

Referring now primarily to FIGS. 6A and 6B, a bus bar module 70 may bepositioned over top of each of the first and second cell stacks 62A,62B. In one non-limiting embodiment, the bus bar modules 70 are cubicshaped and are made of a suitable combination of conductive andinsulating materials. Each bus bar module 70 may be located above one ofthe first and second cell stacks 62A, 62B and can then attached to thebattery cells 58, such as via welding, to electrically connect thebattery cells 58. In one embodiment, the walls 72 of the structuralassembly 64 are sized such that when the first and second cell stacks62A, 62B and the bus bar modules 70 are installed, a flat (or near flat)top surface 88 is provided (see FIG. 6B).

The base 66 may be attached to the structural assembly 64 either beforeor after inserting the first and second cell stacks 62A, 62B. The cover68 (see FIG. 2), however, may be attached to the structural assembly 64after insertion of the contents of the battery assembly 24. The base 66and the cover 68 may be structural plates that are joined to thestructural assembly 64 in a liquid tight manner such as via welding oradhesion.

FIG. 7 illustrates another exemplary battery assembly 124. In thisdisclosure, like reference numbers designate like elements whereappropriate and reference numerals with the addition of 100 or multiplesthereof designate modified elements that are understood to incorporatethe same features and benefits of the corresponding original elements.In this exemplary embodiment, the battery assembly 124 includes one ormore cell stacks 162, a structural assembly 164 including a plurality ofwalls 172, a bus bar module 170 for each cell stack 162, and a resilientenvelope 190.

In one embodiment, the walls 172 of the structural assembly 164 includeflanges 192. The flanges 192 support and act as depth stops forinsertion of the cell stacks 162. Each wall 172 may include one flange192 (e.g., at exterior walls) or two flanges 192 (e.g., at interiorwalls).

In another embodiment, the resilient envelope 190 is disposed around anentire perimeter of the battery assembly 124 to resiliently andhermetically seal the battery assembly 124 relative to the exteriorenvironment EX. The resilient envelope 190 may be a polymer such as highdensity polyethylene (HDPE). Other resilient envelope materials are alsocontemplated. The resilient envelope 190 may exhibit a relatively thinprofile, portions of which may act as a compressible spring. Forexample, portions of the resilient envelope 190 may include corrugations199 (here, disposed at the top portion of the resilient envelope 190)that are compressible to allow the battery assembly 124 to be press fitagainst a mounting surface 194. The battery assembly 124 could then bemounted to the mounting surface 194 using brackets, straps or otherfastening devices. In this way, the battery assembly 124 may exhibitzero clearance relative to the mounting surface 194.

The battery assemblies described by this disclosure provide compactdesigns that leave near zero air spaces inside the assembly. Thisimproves system density and reduces the amount of air available toexpand/contract inside the assembly. Furthermore, the exemplary batteryassemblies provide a packaging solution that reduces the number and sizeof packaging components, substantially eliminates conventional arrayretention components, and substantially eliminates threaded fastenerconnections.

Although the different non-limiting embodiments are illustrated ashaving specific components or steps, the embodiments of this disclosureare not limited to those particular combinations. It is possible to usesome of the components or features from any of the non-limitingembodiments in combination with features or components from any of theother non-limiting embodiments.

It should be understood that like reference numerals identifycorresponding or similar elements throughout the several drawings. Itshould be understood that although a particular component arrangement isdisclosed and illustrated in these exemplary embodiments, otherarrangements could also benefit from the teachings of this disclosure.

The foregoing description shall be interpreted as illustrative and notin any limiting sense. A worker of ordinary skill in the art wouldunderstand that certain modifications could come within the scope ofthis disclosure. For these reasons, the following claims should bestudied to determine the true scope and content of this disclosure.

What is claimed is:
 1. A battery assembly, comprising: a first cellstack including a plurality of battery cells; and a structural assemblyincluding a plurality of walls joined together to form a first pocket toreceive said first cell stack, wherein said first pocket is sized suchthat said first cell stack will only fit into said first pocket whensaid first cell stack is over-compressed, wherein said walls areconfigured to exert a compressive load on said first cell stack as soonas said first cell stack is inserted into said first pocket, and whereinat least one of said walls includes a channel configured to communicatea fluid to thermally condition said first cell stack.
 2. The batteryassembly as recited in claim 1, wherein said plurality of battery cellsare individual cells disposed side-by-side and unbound relative to oneanother.
 3. The battery assembly as recited in claim 2, wherein each ofsaid plurality of battery cells is contiguous with at least one of saidwalls.
 4. The battery assembly as recited in claim 1, comprising asecond cell stack received within a second pocket formed by said walls,wherein said second pocket is sized such that said second cell stackwill only fit into said second pocket when said second cell stack isover-compressed, and wherein said walls are configured to exert acompressive load on said second cell stack as soon as said second cellstack is inserted into said second pocket.
 5. The battery assembly asrecited in claim 4, wherein a center wall of said walls separates saidfirst pocket from said second pocket.
 6. The battery assembly as recitedin claim 5, wherein each of said walls includes a channel configured tocommunicate the fluid to thermally condition said first and second cellstacks.
 7. The battery assembly as recited in claim 1, comprising a busbar module positioned over top of said first cell stack.
 8. The batteryassembly as recited in claim 1, wherein a base is positioned at a bottomof said structural assembly and a cover is positioned at a top of saidstructural assembly.
 9. The battery assembly as recited in claim 1,comprising a resilient envelope disposed around an entire perimeter ofsaid structural assembly.
 10. A method, comprising: applying a force toa cell stack of a battery assembly such that the cell stack isover-compressed; while the cell stack is over-compressed, inserting thecell stack into a pocket of a structural assembly such that walls of thestructural assembly exert a compressive load against the cell stack assoon as the cell stack is inserted into the pocket, wherein, before thecell stack is over-compressed, the cell stack will not fit into thepocket; and directing a flow of fluid through walls of the structuralassembly to thermally condition the cell stack.
 11. The method asrecited in claim 10, wherein the step of applying the force includes:disposing a plurality of battery cells of the cell stack betweenopposing end spacers; and applying a force to the cell stack at theopposing end spacers.
 12. The method as recited in claim 10, wherein thestructural assembly is configured to at least partially enclose the cellstack.
 13. The method as recited in claim 10, comprising: sealing thecell stack of the battery assembly relative to an exterior environmentafter the inserting step.
 14. The battery assembly as recited in claim1, wherein: each of the plurality of battery cells of the first cellstack include first and second faces, side walls, a top, and a bottom,the surface area of each of the side walls is less than the surface areaof each of the first and second faces, and the structural assemblyincludes first and second walls extending along the first cell stacksuch that each of the side walls of each of the plurality of batterycells is contiguous with one of the first and second walls.
 15. Thebattery assembly as recited in claim 1, wherein each of said pluralityof walls includes a plurality of channels configured to communicate thefluid to thermally condition said plurality of battery cells.
 16. Thebattery assembly as recited in claim 5, wherein said structural assemblyincludes a first wall having a first channel of a first cross-sectionalarea and the center wall has a second channel of a secondcross-sectional area greater than said first cross-sectional area. 17.The battery assembly as recited in claim 1, wherein said walls includechannels of different cross-sectional sizes.
 18. The method as recitedin claim 10, wherein the fluid is directed through channels of differentcross-sectional sizes.
 19. The battery assembly as recited in claim 1,wherein said walls are configured to exert a compressive load on saidfirst cell stack immediately after said first cell stack is insertedinto said first pocket.